bamarx300

I have a '99 RX300 with 107,750 miles on it that I recently purchased. I had some work done on it (transfer case replaced) and after they completed the work the Check Engine light came on. They tried to clear it at the transmission shop but it came back on. I had an independent mechanic look at it and he told me that he couldn't interpret the codes (P0125 and P1135) and that I need to take it to Lexus. Well, instead of spending more $ at a dealership I figured that I would try online and found this very helpful sight. Through all of the posts and forums I've read, my conclusion, at this point, was to replace the O2 sensors on both exhaust manifold banks. The definition of the codes are as follows:

P0125:-- "The PCM has determined that the amount of time that it took for the coolant temp. sensor to reach an imput value consisten with closed loop operation was excessive."

P1135:-- "Pedal Position Sensor A Circuit Intermittent."

A: Has anybody had recent experience with these codes and if so what was your ultimate solution to the problem?

B: Would replacing the O2 sensors (both banks/exhaust manifolds) be the solution for this Check Engine Light problem?

Thanks in advance for all imput!!

Lexmex

I also have a 99 RX and the accompanying service manual. I would first disconnect the battery cables for about 30 minutes then drive around and see what codes you get, if any. Clearing the codes (with a scanner) does not clear everything in the ECM's memory. Also see if there is any crud on the red diode on the MAF sensor.

P0125 (Insufficient Coolant Temperature for Close Loop Fuel Control)

DTC Detecting Condition:

After the engine is warmed up, A/F sensor output* does not change when conditions (a), (b) and (c) continue for at least 1.5 minutes.
*: The output value changes at the inside of the ECM only.
(a) Engine speed: 1,500 rpm or more
(b) Vehicle speed: 40~100 km/h (25-62 mph)
(c) Throttle valve does not fully close

Trouble Area:
-Open or short in A/F sensor circuit (bank 1, 2 sensor 1)
-A/F sensor (bank 1, 2 sensor 1)
-ECM

I also notice later on, it talks about checking the harness and connector between the ECM and heated oxygen sensor bank 1, 2 sensor 1.

Thus, it would seem that the MAF, ECM or the connector between the ECM and O2 sensors are the possible culprits.

P1135 (A/F Sensor Heater Circuit Malfunction) (Bank 1 Sensor 1)
(also mentions under this code to refer to P0125 as discussed above)

DTC Detecting Condition:
-When the heater operates, heater current exceeds 8 A (2 trip detection logic)
-Heater current of 0.25 A or less when the heater operates (2 trip detection logic)

Trouble Area:
-Open or in heater circuit of A/F sensor (bank 1, 2 sensor 1)
-A/F sensor (bank 1, 2 sensor 1) heater
-ECM

The only other thing mentions is to check the harness and connector between the EFI main relay and A/F sensor (bank 1, 2 sensor 1) and ECM

Let me know if you want any more info.

bamarx300

Thanks for the info Lexmex. I did check out the MAF and it seemed to be good. I cleaned it anyway, disconnected the battery for about 45 minutes to clear memory, reinstalled MAF, took for test drive and CE light came back on. Do you think O2 sensors could be bad? Anyway to test them? All O2 harnesses looked good under car and under hood-did not check harnesses/connections to ECM. Where is ECM on the '99 RX300?

Lexmex

The ECM is located behind the glove compartment. There is an underpanel beneath the glove compartment you also have to remove.

willie83

My 99 RX300 was throwing codes 0171, 0125, 1135, and 1133. All indicated bank1, sensor1 (firewall side on exhaust manifold before cat). I spent a lot of time researching these and other forums. During that time, I cleaned the MAF, used carb cleaner down the trottle body, and clearing the codes, and they always came back. The car really started running very poorly, no power, rough idle, and crappy gas milage.

OxygenSensors.com has the right A/F ratio sensor for $170.98 with free shipping. Also the sensor socket for $9.95. Denso Part #234-9009. Identical to the original.

You will need a long handle ratchet or extention to get enough torque. Be sure to use some penatrating oil the night before. I also had to put a hose calmp around the split socket to keep it from spreading out! That baby was tight! Once it broke loose, it began to get tight again. I had to work it back and forth a few times and it finally came out. The threads closest to the end looked like they got left behind. I stuck my finger up in there and it felt clean; threads were deep. Put the anti-seize on and screwed the new sensor in and plugged it in. Oh, I forgot to mention what a pain to unplug. Need to get both hands up there to squeeze and pull, probably easier when on a lift.

All said and done for $181.00 and the car runs like new. No new codes after 300 miles.

slick20

is the a/f sensor you mention the same as the front o2 sensor?

willie83

The two sensors on the exhaust manifolds (front and back, same part #) are called air / fuel ratio sensors by Lexus and are before the catalytic convertor and regulate the fuel mixture. They still call the sensor after the cat an O2 sensor. Most people and retailers will refer to all of them as oxygen sensors.

slick20

ok i replaced the front o2 sensor .can't seam to find the rear one. don't see it from the under side of car , also dont see it from top side. has any one done this job themshelves? need to know how to get to the rear one.after cat o2 sensor seems real easy.you guys have been a big help so far .please keep the info comming. thanks.p.s.why are they so expensive? when most o2 sensors are only about $30..00.

Lexmex

If you are not referring to the one after the main catalytic converter below the car, then this is the place. I think you have to sneak underneath the car to get to this place. Start at the two joints that link before the main catalytic converter and take the upwards path and this goes to the back of the engine block where the other part of the exhaust manifold is. This picture will not look exactly like yours since my precats no longer exist and straight pipe is in their place.

The area circled in white is the sensor.

Lexmex

Another sensor is located is between the main catalytic converter and the resonsator directly undeneath the middle portion of the RX.

Lexmex

The most common of the sensors we see is right in the engine bay on the front manifold.

slick20

the 1st pic you posted is the one i was looking for. i'll let you know how i make out.$200.00 ain' t cheap for another one.

Lexmex

No they aren't cheap. I have not had any issues despite all the mods except and occasional P0171 lean error that I erase quickly with my Actron OBDII scanner.

I know that any RX300 turbo project I might do might require O2 eliminators, but we will see.

jimmouse

You can get that piece for about 170 online.

slick20

did you use ramps? any more info on changing this my shelf would be appericiated!!!! 3-24-06 installed new o2 sensor ,cel is out and has been all week end thanks. used ramps made this job accassable . spent 20 min. trying to get connector off.